1. Field of the Invention
The present invention relates to an electronic endoscope apparatus that is used for observing an internal organ in a human body, an object inside a duct, and the like.
2. Description of the Related Art
An electronic endoscope apparatus generally comprises an electronic endoscope with a flexible conduit (or insertion portion) which is inserted into a body or a duct, an image-signal processing device that processes image signals from a CCD which is provided at the distal end of the flexible conduit, and an image-indicating device (or a TV monitor) for monitoring images captured by the CCD. Electronic endoscopes are detachable from the image-signal processing device so that a suitable electronic endoscope can be attached to the image-signal processing device as required. When an electronic endoscope is attached to the image-signal processing device, image signals from the electronic endoscope are amplified and are subjected to A/D conversion, and other types of signal processing are carried out.
The relationship between a received light amount on a CCD and intensity of the signal (voltage) output from the CCD has a linear region and a nonlinear saturated region. A threshold voltage that determines the boundary of the linear region and the saturated region is defined as a saturation voltage. The gain of an amplifier provided in the image-signal processing device for processing image-signals from the electronic endoscope is preferably preset to the level that excludes the saturation level of the amplified image-signals from the tolerance range of the A/D converter. However, since the saturation voltage for the same type of CCD, manufactured in the same lot, is uneven, a suitable gain for the image signals in the image-signal processing device is different for each of the electronic endoscopes attached to the image-signal processing device. Conventionally, the gain of an image-signal processing device is adjusted to a level that covers a CCD with the minimum saturation voltage level, so that an insufficient signal output does not occur for any electronic endoscope.
However, when the gain of an image-signal processing device is preset to the value that covers the minimum saturation voltage for all of the CCDs, image signals from an electronic endoscope with a high output voltage are superfluously amplified by the preset gain and this makes the S/N ratio worse. Namely, reproduced images deteriorate because of noise even though the image signals are obtained at a sufficiently high level and the ability of the CCD to produce quality images is not fully utilized.